Owing to its compact size, low weight and generally high efficiency, switching power supplies have enjoyed ever increasing market adoption in the consumer electronics industry. This is specially so in portable applications where compact size, low weight and battery life are all on top of the list of considerations.
As a first illustration of prior art switching power supply, FIG. 1 illustrates a non-synchronous, single-loop regulated switching converter 1. The single-loop regulated switching converter 1 operates to convert an unregulated DC input 3 into a regulated DC output voltage 5 supplying a power load 4 with power ground 2. A controlled power output transistor 9 drives the power load 4 through a series, parallel network of power inductor 6, power capacitor 7 and passive power diode 8 with the input side 6a of the power inductor 6 connected to the power output transistor 9 and the output side 6b connected to the power load 4. In this single-loop system, the control signal, being the gate voltage of the power output transistor 9, is derived from a feedback control branch having an error amplifier 10 and a pulse width modulation (PWM) controller 11 that turns on or off the power output transistor 9 depending upon the regulated DC output voltage 5 being lower or higher than a “reference” voltage. As the power inductor 6 stores electrical energy with its coil current, the passive power diode 8 free-wheels the inductor current whenever the power output transistor 9 is turned off.
FIG. 2A together with FIG. 2B illustrate a second prior art single-loop synchronous regulated switching converter 20 and some of its related operating signal waveforms. Except for the replacement of the power diode 8 in the single-loop regulated switching converter 1 with a power shunt transistor 21 and its driving inverter 22, the single-loop synchronous regulated switching converter 20 is essentially the same as the single-loop regulated switching converter 1. As the inverter 22 is driven by the output of the PWM controller 11, the single-loop synchronous regulated switching converter 20 operates on synchronously driving the power output transistor 9 and the power shunt transistor 21, in a complementary off/on manner, with the feedback control branch having the error amplifier 10 and the pulse width modulation (PWM) controller 11. This can be seen by comparing the two gate signal waveforms Vgs_Q1 20a and Vgs_Q2 20b of power output transistor 9 and power shunt transistor 21 respectively. To prevent a dangerous condition of shoot-through wherein both transistors 9 and 21 are conducting, a dead time t1 is provided wherein both transistors 9 and 21 are OFF (Ids_Q1 20c=Ids_Q2 20d=0) and a load current Io returns through a built-in parasitic diode (part of power shunt transistor 21, not shown) with forward voltage Vf. The corresponding energy loss is Vf*Io*t1. Additionally, there are energy losses during time intervals t2 and t3 wherein the power output transistor 9 is being switched OFF and the power shunt transistor 21 being switched ON respectively. Thus, the following total energy loss is incurred per switching cycle of the transistors 9 and 21:ELC=Energy loss per switching cycle=0.5*Vf*Io*(t2+t3)+Vf*Io*t1  (1)A highly important measure of performance of a power converter is its overall power efficiency defined as:overall power efficiency=output power/input power  (2)Clearly, the above ELC acts to undesirably lower the overall power efficiency of the single-loop synchronous regulated switching converter 20. As will be shown later, the loss of overall power efficiency becomes rapidly pronounced at lighter load current Io. Hence, a primary object of the present invention is to improve the overall power efficiency.